Nope... You have some item, and it will be replaced with another when duration ends. With 100% chance. But what you'll loot from new item is another story, so finally there is some chance that you'll get protodrake from FHs.
Example: Frenzyheart's jar will be replaced with another one, which contains loot, defined in _loot_template
Frenzyheart Tribe === > http://www.wowhead.com/?item=44717 ===> 1 week ===> http://www.wowhead.com/?item=44718 ===MAYBE===> http://www.wowhead.com/?item=44719
Oracles ===> http://www.wowhead.com/?item=39878 ===> 1 week ===> http://www.wowhead.com/?item=39883 ===MAYBE===> http://www.wowhead.com/?item=44707
Anyway, there is only 2 items atm, so IMHO new script event on destroy will be the best solution.